Along with the rapidly spreading use of the Internet and the expansion of business communication networks, the demand for communication is dramatically increasing with the consequence that the capacities of relayed networks are falling short of the requirement. In this connection, optical fiber networks are pressed for further increases in speed and capacity.
A transmission technique known as dense wavelength division multiplex (hereinafter abbreviated to DWDM), which allows an optical fiber to propagate many wavelengths, is attracting note and coming into growing use as a technique that makes possible a big leap in the expansion of transmission capacities for relay networks owned by communications operators especially in urban areas.
The DWDM transmission technique allows transmission of a plurality of optical signals differing in wavelength over a single optical fiber; it is a technique that makes possible a big leap in the expansion of transmission capacity of a single optical fiber. Particularly in next-generation large capacity and high speed systems for transmission at such a high speed as 10 Gb/s or even 40 Gb/s, the influence of polarization mode dispersion (hereinafter abbreviated to PMD) is at work in addition to usual dispersion characteristics, inviting a further increase in coefficient s contributing to waveform deterioration during transmission. For this reason, the problem of how to reduce this polarization mode dispersion has come to take on extreme importance.
Polarization mode dispersion is a phenomenon that occurs when an optical wave comes incident between one optical axes {a fast direction (y-polarized) or a slow direction (x-polarized)} and another (e.g. at 45°), equivalent to each other generated by the birefringence within the optical fiber, a difference in refractive index between two mutually orthogonal polarized components gives rise to a difference in group delay time between the two polarizations and thereby widens the optical pulse. The magnitude of polarization mode dispersion is represented by the difference in group delay time in the lengthwise direction of the orthogonal polarization arising from the elliptical deformation of the core and traces of anisotropic stresses (including those due to lateral pressure, bending, torsion, tension and thermal stress due to temperature variation), and this difference in group delay time is defined as PMD (unit: ps) by the International Telecommunication Union (abbreviated to ITU).
The quotient of the division of that polarization mode dispersion by the square root of the distance is defined to be the polarization mode dispersion (PMD) coefficient (unit: ps/√km). This difference in group delay time is also referred to as DGD (differential group delay; unit: ps).
Whereas optical fibers should be spliced to each other in architecting a communication system, there are a few kinds of splicing methods for that purpose The fusion splicing method, which excels in reliability and connecting performance among them, has a disadvantage of taking a longer time per splice than other methods. What has been developed to increase the density of and reducing the length of time taken to fusion-splice these cables is the optical fiber-tape. For instance in J. Kohtala, J. Tanskanen, P. Fickling, and M. Eriksson, “A High Speed Coating Process for Optical Fiber Ribbon”, in Proceedings of International Wire Cable Symposin. '91 (St. Louis, U.S.A.), 1991, pp. 550–555”, its structure and fabrication method are described.
In the general structure of an optical fiber tape optical fiber are arranged in a lateral row and integrated. For example, an optical fiber consists of an optical fiber glass as such measuring 125 μm in diameter coated by a covering layer and a coloring layer of a few μm to make the external diameter 250 μm. Available optical fiber include quartz-based single-mode optical fibers, quartz-based multi-mode optical fibers and dispersion shift optical fibers, basically composed of silica glass or germanium doped glass.
Two methods are available for integration, one of adhering adjoining optical fibers and the other of cladding adjoining optical fibers with a tape layer. The number of optical fibers accommodated in a single optical fiber tape is prescribed to be 2, 4, 5, 6, 8, 10 or 12 according to JISC 6838.
Many such optical fiber tapes are put together into a cable form to configure an optical fiber cable. The advancement of optical amplification techniques in recent years has made possible non-regenerative relay transmission for hundreds to thousands of kilometers by using a single mode optical fiber or a dispersion shift optical fiber.
However, since the deterioration of optical signals by polarization mode dispersion poses a problem in such long distance transmission, especially in a DWDM transmission system, realization of optical fiber cables susceptible to little polarization mode dispersion is in keen demand. In order to achieve optically amplified transmission over a long distance of hundreds to thousands of kilometers at a transmission speed of several Gb/s to several tens of Gb/s, the polarization mode dispersion coefficient of the optical fiber cable should be kept at no more than 0.3 ps/√km, more preferably at no more than 0.2 ps/√km.
For these reasons, optical fiber tapes are also required to have superior PMD characteristics. However, there is a problem that, even if optical fibers themselves are superior in polarization mode dispersion, their integration into a tape invites deterioration in polarization mode dispersion. Also, whereas a tape-shaped optical fiber usually accommodates a plurality of optical fibers, there is a further problem of differences in PMD characteristics among the individual optical fibers. Thus, in the prevailing state, the polarization mode dispersion of optical fiber tapes has been inadequate especially for use in a DWDM system for long distance transmission.
The present invention is intended to provide an optical fiber tape core adaptable to transmission at high speed of 10 Gb/s or even 40 Gb/s.